1. Field of the Invention
The present invention relates to a process for ultrafiltration of stabilized emulsions, for example of used cutting oils, and more particularly to such a process consisting in introducing a small quantity of salt into the emulsion with a view to destabilizing it, that is to say without preliminary separation of the water and of the oil before its delivery to the ultrafiltration membrane.
2. Description of the Related Art
A cutting oil is a mixture of mineral oil, of surface-active agents, of cosurfactants and of many various additives (bactericides, extreme pressure agents, lubricants, corrosion inhibitors, wetting agents and the like). This combination of constituents which are emulsifiable in all proportions with water is commonly employed at concentrations which vary from 1 to 10% of oil per 90 to 99% of water. Cutting oil emulsions are employed in all operations for machining and shaping metals and cutting stones in order to provide the following functions at the cutting tool:
lubrication and reduction in friction, PA1 cooling, PA1 reduction in wear and corrosion, PA1 removal of the impurities (swarf, dust etc.). PA1 Treatment processes using a thermal route: Two types of treatment can be distinguished; the simplest one consists of direct burning of the spent emulsion; the other type is based on an evaporation. The aqueous phase is thus evaporated off and the oil is recovered at the end of the operation. These two methods are adapted to all types of cutting fluid but have a major disadvantage, namely a very high energy consumption. PA1 Physicochemical treatment processes: These processes are based on a destabilization of the spent oil-water emulsion, which is often referred to as "breaking the emulsion". This breaking is generally obtained by the action of chemical reactants of acidic, salt or polyelectrolyte type; the subsequent separation of the water and of the oil is generally carried out by simple phase separation. An example of this type of breaking process is described in document FR-A-2,656,812. Although resulting in good separation efficiencies after quite long phase separation periods, these processes exhibit two major disadvantages. The first disadvantage is related to the large quantities of reactants to be employed and is reflected in the substitution of an acidic or saline pollution for an initial organic pollution. The second major disadvantage of these techniques is that the breaking is comparable to a chemical reaction and is therefore found to be stoichiometric, that is to say that it requires an optimum dosage of the reactants employed. It is therefore essential to carry out tests as during a flocculation in order to determine beforehand the optimum concentrations of salts, acids or polyelectrolytes to be employed. PA1 Ultrafiltration processes: In order to separate the oil from the water by ultrafiltration, the emulsion containing cutting oil droplets with a diameter of less than 5 .mu.m is circulated through an ultrafilter equipped with a water-permeable porous membrane whose pores have a diameter of approximately 100 .ANG.. Treatment processes employing ultrafiltration exhibit undoubted advantages. They consume only little energy, the treatment plants are small in size and, after treatment, the water is free from cutting oil. In addition, no human maintenance is needed continuously, as in the case of the physicochemical processes and consequently they can be easily automated and this is a considerable advantage in the present context. However, this ultrafiltration technique involves limitations which are related especially to the viscosity of the emulsion which will have a direct effect on the flow of the permeate and the formation, in the course of time, of a so-called "polarization" layer on the membrane which is produced by the gradual accumulation of the oil droplets. When this layer contains from 30 to 40% of oil, it has the consistency of a particularly viscous whitish gel. The blocking thus obtained causes a very appreciable decrease in the water-permeability of the membrane, that is to say a reduction in the flow of permeate passing through it and the elimination of the capillary separation action in the blocked regions of the membrane, which is reflected in a leakage of oil and poor oil-water separation. To overcome this major disadvantage of the ultrafiltration method it is known to lower the viscosity of the emulsion either by diluting it or by completely or partially breaking the emulsion by introducing salts, organic compounds or acids or simply by diluting the emulsion. In the prior art this chemical destabilization is accompanied by a phase separation preceding or following the ultrafiltration. This type of destabilization prior to the ultrafiltration consists in treating two different phases, the oil separated off beforehand and the residual emulsion to be ultrafiltered, the viscosity of which has thus been reduced.
These emulsions operate in a closed circuit on machine tools until they lose their effectiveness over some months because of a slow bacterial degradation and because of being contaminated with impurities. They must consequently be replaced at regular intervals. The organic pollution caused by a direct discharge of these spent emulsions into natural surroundings can be damaging to the environment. The problem of the treatment of these spent cutting oil emulsions must therefore be faced.
Stabilized emulsions cannot be treated by conventional methods of separation which are employed for unstabilized emulsions, namely: phase separation, flotation or coalescence with a particle or fibre bed or hydrocyclones, because the oil droplets are too small to be capable of being separated by gravity separation. In addition, the presence of the surface-active agents and of the cosurfactants prevents any coalescence of the oil droplets because of the existence of an electrical and/or mechanical barrier.
The treatment methods which are employed at present can be classified into three categories:
These improvements in the prior art for the separation of stable emulsions by ultrafiltration nevertheless do not make it possible to develop these ultrafiltration processes economically on an industrial scale because two stages are needed to improve the flow of permeate through the membrane, one of these being destabilization by breaking the emulsion. Furthermore, an oil pollution is replaced with a considerable saline pollution in the case of which no favourable solution is available. In addition, human supervision is necessary and a process of this type can be only partially automated.
The present invention is therefore aimed at improving the ultrafiltration processes using a membrane in order to improve their performance, that is to say to increase the flow of permeate, to limit the problems of blocking and of formation of the polarization layer, and to reduce the membrane areas used at present, but to do this in a single stage, without preliminary destabilization by breaking the emulsion into two phases, of "oil" and "lower viscosity emulsion".